This chapter introduces the reader to some of the great constants of Gottfried Wilhelm Leibniz’s metaphysics as they appear in the first works of his philosophical maturity; the unity, indivisibility, and activity of substances, for instance, are themes which are fully present in these works and never disappear from subsequent writings. In the Discourse on Metaphysics and the subsequent correspondence with Antoine Arnauld, Leibniz argues that Descartes goes astray in his metaphysics through his failure to think through the implications of Aristotle’s teachings seriously. One weakness of the logicist strategy is that it leaves at least one of Leibniz’s doctrines concerning causality shrouded in mystery. Leibniz’s final objection to occasionalism is that it leads to the Spinozistic heresy that God is the only substance. Leibniz’s attempt to derive his metaphysics in this way is discussed in the light of the criticism that it conflates the level of concepts in God with the level of substances in the world.